A. Field of Invention
The present invention is related to wireless communication systems, and more particularly, to a method and system of controlling forward frame error rates and frame error patterns in a CDMA mobile test setup. It is particularly useful in evaluating a mobile station's vocoder response to specified frame error rates and frame error patterns.
B. Description of Related Art
Voice coders, or “vocoders,” (or codecs) are used to convert voice signals into digital data packets or data streams that may then be transmitted over a digital data communication channel. In a typical wireless phone or data service, such as the widely used CDMA and GSM cellular phone systems, vocoders are designed using various speech coding algorithms (e.g., RCELP (relaxed code-excited linear predictive coding), ACELP (algebraic code excited linear prediction), etc.) which have been standardized in the CDMA networks (EVRC (enhanced variable rate coder, as described in IS-127), Q13 (as described in IS-733), and SMV (selectable mode vocoder, currently under development)) and GSM networks (EFR (enhanced full rate), AMR (adaptive multi-rate)). The performance of the vocoder is rated in terms of how well the vocoder reproduces the analog voice-band signal. Typical rating systems used in the wireless communication industry include Perceived Equivalent Speech Quality (PESQ), as further defined in ITU specification P.862, Mean Opinion Score (MOS), as further defined in ITU specification P.80, and Perceptual Analysis Measurements System (PAMS) developed by British Telecom, which are objective standardized methods of voice quality scoring. In addition, TTY coder performance may be evaluated using a character error rate.
Vocoder performance evaluations also take into account how well the vocoder performs in the presence of errors caused by the communication channel. In a typical wireless communication system, the communication channel that is the source of most of the impairments is between a base transceiver station (BTS) and a mobile subscriber unit, such as a cell phone. Typically, the errors in a wireless system are caused by various channel impairments such as additive noise, power fluctuations (most commonly due to Raleigh fading), radio interference, frequency response variations, nonlinearities, etc. The wireless communication system utilizes error control coding to detect and correct errors within the data frames, but the ability to correct errors is limited. In the event the error correcting capacity of the code is exceeded, the entire data frame is marked as an error and is discarded. This is referred to as a frame error, and the number of frame errors compared to the total number of frames is referred to as the frame error rate.
Numerous test configurations and procedures have been specified by various standards organizations. These traditional methods of measuring vocoder performance, as described in various CDMA Standard test documents, typically use a complex Raleigh fading channel model and converter to closely model the channel characteristics. The Raleigh fader model is designed to check the base station and mobile ability to use forward power control to control the frame error rate (FER) at a low set point in the presence of fading. This testing model, however, has limitations in its use for determining characteristic behaviors of radios in the presence of frame error excursions beyond the low set point.
Consequently, a test configuration that allows more precise control of frame error rates and frame error patterns that overcomes the current limitations is needed.